Numerous kinds of vehicles having retractable jacks for stabilization and/or lifting are known in the art and are used in a wide range of applications. Typically, the stabilizing jacks are hydraulically operated and are moveable between retracted and extended positions. When in the retracted position, the stabilizing jacks are out of the way and allow the vehicle to move about without interference from the jacks. When in the extended position, the stabilizing jacks contact the ground and support at least a portion, if not the entirety, of the vehicle. In certain applications, the jacks may be used merely to stabilize the vehicle, whereas in other applications, the jacks my lift all or a portion of the vehicle to level the vehicle or otherwise position the vehicle in a desired attitude.
While such stabilizing jack systems may be manually controlled, many jack systems are partially- or fully-automated, and use a jack deployment system to automatically extend or deploy the jacks until they provide the desired degree of lift or stabilization. A typical jack deployment system uses a ground sensing system to first sense or detect when the jacks have contacted the ground. Thereafter, the jack deployment system may use an attitude control system to further extend the jacks until the vehicle has achieved the desired attitude (e.g., level or some other attitude).
While the ground sensing systems used by such jack deployment systems are generally capable of determining when the jacks have contacted the ground, they are not without their problems. For example, one type of ground sensing system utilizes a proximity sensor contained within the jack housing. As the jack contacts the ground, a reference element inside the sensor housing moves. The proximity sensor detects the movement of the reference element and provides a suitable indication to the jack deployment system that the jack has contacted the ground. While such proximity sensor systems are generally inexpensive and easy to service, they are prone to failure as a result of clogging caused by grease and/or mud build-up in and around the jack housing.
Another type of ground detection system uses pressure sensors or transducers to detect the hydraulic pressure inside the jack cylinder. When the jack contacts the ground, the hydraulic pressure in the cylinder increases, thereby providing the jack deployment system with an indication that the jack has contacted the ground. While such pressure sensing systems are free of many of the problems associated with proximity sensor systems, they have proven to be no panacea, and also suffer from various drawbacks and disadvantages.
For example, most hydraulically operated jack systems utilize one or more holding valves to hold the jacks at certain positions. The holding valves are adjustable and are usually set-up by a maintenance technician to provide the desired holding characteristics. However, if the holding valve pressure is set too high, the ground detection system may provide a false indication of ground contact. That is, the pressure required to overcome the jack holding valve may be sufficiently high so as to cause the ground contact system to interpret the high pressure as ground contact when in fact the jack has yet to contact the ground.
Still another problem is that the back pressure caused by the jack holding valve typically varies depending on the temperature of the hydraulic fluid. When the fluid is cold, the pressure required to overcome the valve will usually increase, again leading to the possibility of a false ground contact signal. Yet another disadvantage associated with pressure sensing systems is that the various components are relatively expensive and may be difficult to service or may create other maintenance problems.
Besides the reliability and maintenance issues that are associated with the various kinds of ground detection systems, the failure of the ground sensing system to detect when the jacks have contacted the ground may result in other problems, such as an unsafe vehicle attitude or even vehicle inversion. That is, absent a ground detection signal, the jack deployment system may continue to extend the jack even though the jack has already contacted the ground. Unchecked jack extension will almost certainly result in an unsafe vehicle attitude and may even cause the vehicle to tip or roll over, particularly if the vehicle is on uneven or sloped ground. In other failure modes, a “false positive,” i.e., an indication of jack ground contact when in fact none as occurred, may prevent the jack deployment system from further extending the jacks, thereby preventing the vehicle from achieving the desired attitude or degree of stabilization.